Summary: The crystal structure of epistilbite ((Ca2·59Na1·06K0·10) (Al6·29Si17·71O48).15·74H2O; C2/m; a = 9·08, b = 17·74, c = 10·25±0·01 Å; β = 124·54±0·05°) was determined by 3-D least-squares methods. The alumino-silicate framework is composed of 4-, 5-, and 8-membered rings of tetrahedra. There are two sets of intersecting channels each defined by 8-membered rings. The (Ca, Na) atoms in the channels are in contact with seven water molecules and two oxygen atoms giving a coordination of nine at a cut-off distance of 2·77 Å. The average tetrahedral distance is lower than for anhydrous framework alumino-silicates but consistent with some other zeolite structures. Partial Al-Si ordering exists with one tetrahedron occupied preferentially by aluminum. This tetrahedron contains the two oxygen atoms that are coordinated with the (Ca, Na) atom.